Lithographic apparatus and device manufacturing method

ABSTRACT

A map of the surface of a substrate is generated at a measurement station. The substrate is then moved to where a space between a projection lens and the substrate is filled with a liquid. The substrate is then aligned using, for example, a transmission image sensor and, using the previous mapping, the substrate can be accurately exposed. Thus the mapping does not take place in a liquid environment.

[0001] This application claims priority from European patentapplications EP 02257822.3, filed Nov. 12, 2002, and EP 03253692.2,filed Jun. 11, 2003, both herein incorporated in their entirety byreference.

FIELD

[0002] The present invention relates to immersion lithography.

BACKGROUND

[0003] The term “patterning device” as here employed should be broadlyinterpreted as referring to means that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate; theterm “light valve” can also be used in this context. Generally, the saidpattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such a patterning device include:

[0004] A mask. The concept of a mask is well known in lithography, andit includes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

[0005] A programmable mirror array. One example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. An alternative embodiment of a programmable mirror arrayemploys a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuation means. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors; in this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-addressable mirrors.The required matrix addressing can be performed using suitableelectronic means. In both of the situations described hereabove, thepatterning device can comprise one or more programmable mirror arrays.More information on mirror arrays as here referred to can be gleaned,for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193,and PCT patent applications WO 98/38597 and WO 98/33096, which areincorporated herein by reference. In the case of a programmable mirrorarray, the said support structure maybe embodied as a frame or table,for example, which may be fixed or movable as required.

[0006] A programmable LCD array. An example of such a construction isgiven in U.S. Pat. No. 5,229,872, which is incorporated herein byreference. As above, the support structure in this case may be embodiedas a frame or table, for example, which may be fixed or movable asrequired.

[0007] For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

[0008] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, thepatterning device may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g. comprising one or more dies) on a substrate (siliconwafer. LCD, mask etc) that has been coated with a layer ofradiation-sensitive material (resist). In general, a single wafer willcontain a whole network of adjacent target portions that aresuccessively irradiated via the projection system, one at a time. Incurrent apparatus, employing patterning by a mask on a mask table, adistinction can be made between two different types of machine. In onetype of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionat one time; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatus—commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

[0009] In a manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

[0010] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, incorporated herein by reference in their entirety.

[0011] The lithographic industry is constantly trying to reduce featuresizes on silicon substrates in order to manufacture ever more complexintegrated circuits. The feature sizes are limited by the effect ofdiffraction and thus the resolution of a particular system of numeralaperture NA using a wavelength λ is: $W = {k\quad \frac{\lambda}{NA}}$

[0012] where k is a pre-factor. The numerical aperture NA is n sin θwhere n is the refractive index of the transmissive substance.

[0013] Hence to decrease the resolution, the wavelength can either bereduced or the numerical aperture increased. It has been proposed toimmerse the substrate in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system).

[0014] However, submersing the substrate or substrate and substratetable in a bath of liquid (see for example U.S. Pat. No. 4,509,852,hereby incorporated in its entirety by reference) may mean that there isa large body of liquid that must be accelerated during a scanningexposure. This may require additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

[0015] One of the solutions proposed is for a liquid supply system toprovide liquid in a localized area between the final element of theprojection system and the substrate (the substrate generally has alarger surface area than the final element of the projection systems).One way which has been proposed to arrange for this is disclosed in PCTpatent application WO 99/49504, hereby incorporated in its entirety byreference. As illustrated in FIGS. 4 and 5, liquid is supplied by atleast one inlet IN onto the substrate, preferably along the direction ofmovement of the substrate, relative to the final element, and is removedby at least one outlet OUT after having passed under the projectionsystem. That is, as the substrate is scanned beneath the element in a −Xdirection, liquid is supplied at the +X side of the element and taken upat the −X side. FIG. 4 shows the arrangement schematically in whichliquid is supplied via inlet IN and is taken up on the other side of theelement by outlet OUT which is connected to a low pressure source. Inthe illustration of FIG. 4 the liquid is supplied along the direction ofmovement of the substrate relative to the final element, though thisdoes not need to be the case. Various orientations and numbers of in andout-lets positioned around the final element are possible, one exampleis illustrated in FIG. 5 in which four sets of an inlet with an outleton either side are provided in a regular pattern around the finalelement to form a liquid reservoir.

SUMMARY

[0016] Immersion lithography is an embryonic technology and there remainmany problems in its practical application. This patent application isconcerned in particular with alignment and leveling of a substrate.Conventionally alignment and leveling is performed with the substrate inthe field of view of the projection system (i.e. at an exposurestation). However there is not a lot of space for alignment or levelmeasurement apparatus in and around an immersion liquid reservoir so theadaptation is likely to be complex or the accuracy can be compromised.Furthermore, the presence of liquid near the alignment and levelmeasurement apparatus can degrade the performance of the apparatus.

[0017] Accordingly, it may be advantageous to provide, for example, amethod and apparatus for accurately aligning and/or leveling a substratein an immersion lithography apparatus.

[0018] According to an aspect, there is provided a lithographicprojection apparatus comprising:

[0019] a support structure configured to hold a patterning device, thepatterning device configured to pattern a beam of radiation according toa desired pattern;

[0020] a substrate table configured to hold a substrate;

[0021] a projection system configured to project the patterned beam ontoa target portion of the substrate;

[0022] a liquid supply system configured to provide a liquid in a spacebetween the final element of said projection system and said substrate;and

[0023] a measurement system configured to measure, not through saidliquid, locations of points on said substrate.

[0024] The position of points on the substrate are thus measured withoutthe presence of liquid and, in an embodiment, outside the immersionsystem. Alternatively, the measurements could take place while a targetportion of the substrate is submerged in liquid, i.e. the measurementstake place through liquid, but not the same liquid as supplied by theliquid supply system to fill a space between the final element of theprojection system and the substrate. The position of points on thesubstrate would therefore be measured with liquid between themeasurement system and the substrate, the liquid would then be removedbefore moving the substrate (and substrate table) to the focal point ofthe projection system where the liquid supply system would supply liquidto fill a space between the final element of the projection system andthe substrate prior to exposure taking place. A second liquid supplysystem may be present in the vicinity of the measurement system.

[0025] A possible advantage is that there is better flow in the liquidreservoir because the measurement system is no longer in or around thereservoir crowding the projection system and the performance of themeasurement system is not degraded by the presence of liquid.Furthermore smooth flow conditions in the liquid reservoir are desiredas there is no change in the apparatus leading to rough edges. Usingthis method, measurement systems not specifically adapted for immersionlithography can be used without complex adaptation. A further advantageof this measurement system is that any improvements to such measurementsystems used outside of the immersion lithography field can easily andautomatically be incorporated into the immersion system.

[0026] The measurement system, in an embodiment, comprises an alignmentsystem configured to measure the location (in the x, y and R_(z)directions) of a plurality of alignment marks on said substrate.According to an embodiment, said substrate table has a reference andsaid measurement system measures the location of said reference notthrough said liquid of said supply system. The location of the alignmentmarks may in an embodiment be measured relative to said reference onsaid substrate table to enable a map of alignment marks relative to thereference to be built up.

[0027] According to an embodiment, the measurement system comprises alevel sensor configured to measure the height and/or tilt (i.e.measuring in the z, R_(x) and R_(y) directions) of points on saidsubstrate. Thus, level measurement of the substrate, which isconventionally undertaken “on-the-fly” at the exposure station, can beachieved outside the liquid reservoir.

[0028] In an embodiment, the lithographic projection apparatus can havean exposure station at which said substrate may be exposed and aseparate measurement station, said measurement system being provided atsaid measurement station and said substrate table being movable betweensaid exposure and measurement stations. Furthermore, there can be aplurality of substrate tables, each movable between an exposure stationand a measurement station. While one substrate table is being mapped, asecond substrate table can be exposed. Substrate throughput maytherefore be increased, making the apparatus more efficient andimproving the cost of ownership.

[0029] According to an embodiment, said reference is a transmissionimage sensor.

[0030] In an embodiment, the alignment system measures displacement intwo linear perpendicular directions and rotation within the planedefined by the two perpendicular directions.

[0031] According to a further aspect, there is provided a devicemanufacturing method comprising:

[0032] providing a liquid in a space between a final element of aprojection system and a substrate;

[0033] measuring the locations of points on a substrate using ameasurement beam projected from a measurement system but not projectedthrough said liquid; and

[0034] projecting a patterned beam of radiation onto a target portion ofthe substrate using the projection system.

[0035] Although specific reference may be made in this text to the useof the apparatus described herein in the manufacture of ICs, it shouldbe explicitly understood that such an apparatus has many other possibleapplications. For example, it may be employed in the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, liquid-crystal display panels, thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “reticle”, “wafer”or “die” in this text should be considered as being replaced by the moregeneral terms “mask”, “substrate” and “target portion”, respectively.

[0036] In the present document, the terms “radiation” and “beam” areused to encompass all types of electromagnetic radiation, includingultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or126 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich:

[0038]FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

[0039]FIG. 2 depicts a detail of a lithographic projection apparatusaccording to an embodiment of the invention;

[0040]FIG. 3 depicts the same details of the lithographic projectionapparatus as FIG. 2 at a different stage in the exposure processaccording to an embodiment of the invention;

[0041]FIG. 4 depicts an alternative liquid supply system according to anembodiment of the invention; and

[0042]FIG. 5 is an alternative view of the liquid supply system of FIG.4 according to an embodiment of the invention.

[0043] In the Figures, corresponding reference symbols indicatecorresponding parts.

DETAILED DESCRIPTION

[0044]FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

[0045] a radiation system Ex, IL, for supplying a projection beam PB ofradiation (e.g. UV radiation), which in this particular case alsocomprises a radiation source LA;

[0046] a first object table (mask table) MT provided with a mask holderfor holding a mask MA (e.g. a reticle), and connected to firstpositioning means for accurately positioning the mask with respect toitem PL;

[0047] a second object table (substrate table) WT provided with asubstrate holder for holding a substrate W (e.g. a resist-coated siliconwafer), and connected to second positioning means for accuratelypositioning the substrate with respect to item PL;

[0048] a projection system (“lens”) PL (e.g. a refractive lens system)for imaging an irradiated portion of the mask MA onto a target portion C(e.g. comprising one or more dies) of the substrate W.

[0049] As here depicted, the apparatus is of a transmissive type (e.g.has a transmissive mask). However, in general, it may also be of areflective type, for example (e.g. with a reflective mask).Alternatively, the apparatus may employ another kind of patterningdevice, such as a programmable mirror array of a type as referred toabove.

[0050] The source LA (e.g. a laser-produced or discharge plasma source)produces a beam of radiation. This beam is fed into an illuminationsystem (illuminator) IL, either directly or after having traversedconditioning means, such as a beam expander Ex, for example. Theilluminator IL may comprise adjusting means AM for setting the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally comprise various other components, such as an integratorIN and a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

[0051] It should be noted with regard to FIG. 1 that the source LA maybe within the housing of the lithographic projection apparatus (as isoften the case when the source LA is a mercury lamp, for example), butthat it may also be remote from the lithographic projection apparatus,the radiation beam which it produces being led into the apparatus (e.g.with the aid of suitable directing mirrors); this latter scenario isoften the case when the source LA is an excimer laser. The currentinvention and claims encompass both of these scenarios.

[0052] The beam PB subsequently intercepts the mask MA, which is held ona mask table MT. Having traversed the mask MA, the beam PB passesthrough the projection system PL, which focuses the beam PB onto atarget portion C of the substrate W. With the aid of the secondpositioning means (and interferometric measuring means IF), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning means can be used to accurately position the mask MAwith respect to the path of the beam PB, e.g. after mechanical retrievalof the mask MA from a mask library, or during a scan. In general,movement of the object tables MT, WT will be realized with the aid of along-stroke module (course positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. However, inthe case of a wafer stepper (as opposed to a step-and-scan apparatus)the mask table MT may just be connected to a short stroke actuator, ormay be fixed in the XY plane.

[0053] The depicted apparatus can be used in two different modes:

[0054] 1. In step mode, the mask table MT is kept essentiallystationary, and an entire mask image is projected at one time (i.e. asingle “flash”) onto a target portion C. The substrate table WT is thenshifted in the x and/or y directions so that a different target portionC can be irradiated by the beam PB;

[0055] 2. In scan mode, essentially the same scenario applies, exceptthat a given target portion C is not exposed in a single “flash”.Instead, the mask table MT is movable in a given direction (theso-called “scan direction”, e.g. the y direction) with a speed v, sothat the projection beam PB is caused to scan over a mask image;concurrently, the substrate table WT is simultaneously moved in the sameor opposite direction at a speed V=Mv, in which M is the magnificationof the projection system PL (typically, M=¼ or ⅕). In this manner, arelatively large target portion C can be exposed, without having tocompromise on resolution.

[0056] In FIG. 2 the substrate table WT is at a measurement stationwhere alignment and/or level measurement take place. The substrate tableis provided with a reference F1, sometimes referred to as a fiducial,which can comprise a plate etched through with a pattern correspondingto a standard alignment mark underneath which is a radiation sensor,also known as a transmission image sensor, responsive to radiation. Atthe measurement station, the substrate table WT is moved to detect thereference F1 using an alignment system within the measurement system 30and then to detect the alignment marks on the substrate W therebyenabling the location (in directions x, y and R_(z)) of the substratealignment marks to be found. In an embodiment, the location of thealignment marks are measured and determined relative to the reference.

[0057] Level measurement of the substrate then occurs at the measurementstation. In order to measure the level of the substrate, a leveling beam(projected from the measurement system 30) can be used that traverses afirst grating prior to reflection by the substrate W. A second gratingis then placed in the path of the leveling beam after reflection by thesubstrate W. The extent to which the images of the first and secondgratings coincide is measured by a level measurement sensor and isdetermined by the height and/or tilts of the substrate W (the z, R_(x)and R_(y) coordinates are thus determined). For a further description oflevel measurement of substrates reference is made to European patentapplication EP 0502583. Hence, using data from the alignment of thesubstrate and the level measurement of the substrate, a map of thesubstrate can be generated.

[0058] As shown in FIG. 3, substrate table WT is then moved to theseparate exposure station where a liquid supply 18 is provided to supplyliquid (e.g. water) to a space between the projection system PL and thesubstrate table WT to form a liquid reservoir 10. In this example, thereservoir 10 forms a contactless seal to the substrate around the imagefield of the projection system PL so that liquid is confined to fill aspace between the substrate surface and the final element of theprojection system PL. A seal member 12, positioned below and surroundingthe final element of the projection system PL, borders the reservoir 10and comprises the liquid supply 18. The seal member 12 extends a littleabove the final element of the projection system and has an innerperiphery that at the upper end closely conforms to the step of theprojection system or the final element thereof and may, e.g., be round.At the bottom, the inner periphery closely conforms to the shape of theimage field, e.g., rectangular though this need not be the case. Liquidis brought into the space below the projection system and within theseal member 12 and the liquid level rises above the final element of theprojection system PL so that a buffer of liquid is provided.

[0059] A gas seal 16, formed between the bottom of the seal member 12and the surface of the substrate W, confines the liquid in thereservoir. The gas seal is formed by gas, e.g. air or synthetic air butin an embodiment, N₂ or another inert gas, provided under pressure viainlet 15 to the gap between seal member 12 and the substrate W andextracted via first outlet 14. An overpressure on the gas inlet 15,vacuum level on the first outlet 14 and geometry of the gap are arrangedso that there is a high-velocity gas flow inwards that confines theliquid.

[0060] In an embodiment, the liquid reservoir defined by inlet(s) IN andoutlet(s) OUT as shown in FIGS. 4 and 5 can be similarly applied. Insuch a case, a measurement station can be provided as well as anexposure station comprising inlet(s) IN and outlet(s) OUT.

[0061] To ascertain the exact position of the substrate table WT at theexposure station the reference F1 is scanned in three dimensions throughthe aerial image of an alignment mark on the mask MA. The maximum signalis returned when the reference is aligned with the image of the mark onthe mask in the plane of best focus. Using the map of the substrate Wgenerated at the measurement station, the location, height and/or tiltof positions on the substrate W are therefore known. In order to trackthe movements of the substrate table WT, suitable position measurementsdevices can be used such as an interferometer beam projected towards oneor more sides of the substrate table WT. A particular point on thesubstrate table can be placed at the focal point of the projectionsystem PL and exposure of a target portion C of the substrate W can takeplace.

[0062] Once exposure of the substrate W is completed it is then removedfor further processing and a new substrate placed on substrate table WT.The substrate table with the new substrate returns to the measurementstation and the process can be repeated.

[0063] Prior to the substrate table WT leaving the exposure station, theliquid reservoir can be emptied, for example in the case shown in FIGS.2 and 3, by reducing the gas inlet pressure and allowing the liquid tobe sucked out by the vacuum system or, for example in the case shown inFIGS. 4 and 5, by discontinuing flow of liquid onto the substratethrough inlet IN and allowing the liquid to be sucked out by outlet OUT.

[0064] To ascertain the exact position of the substrate table WT, theposition of the transmission image sensor described above can be sensedthrough the liquid, or alternatively not through the liquid and acorrection applied.

[0065] According to an embodiment, there are at least two substratetables, each bearing a reference, and while one substrate table is atthe measurement station the other is at the exposure station. Thesubstrate tables are movable between an exposure station and ameasurement station.

[0066] Instead of using the reference mark F 1 and the projection systemto align the substrate, off-axis measurement can be used. The referencemark F1 can be aligned using another system near the projection systemPL. Alternatively, a different reference and a different system, forexample one with an axis perpendicular to the projection axis of theprojection system can be used. Further description of such off-axismeasurement can be found in European patent application publication EP0906590.

[0067] Alternatively, if the substrate table is above the projectionsystem (i.e. the projection system is upside down compared to FIG. 1)the liquid in liquid reservoir 10 may not need to be completely removedand could just be refilled as necessary.

[0068] In an alternative detection embodiment, there is no separatemeasurement station. Detection and measurement of an alignment marktakes place at the exposure station but with no liquid in reservoir 10.The liquid reservoir 10 is then filled up and exposure takes place.Similarly level measurement can take place at the exposure station withno liquid in reservoir 10. These measurements can be either off-axis oron-axis.

[0069] While specific embodiments of the invention have been describedabove, it will be appreciated that the invention may be practicedotherwise than as described. The description is not intended to limitthe invention.

1. A lithographic projection apparatus comprising: a support structureconfigured to hold a patterning device, the patterning device configuredto pattern a beam of radiation according to a desired pattern; asubstrate table configured to hold a substrate; a projection systemconfigured to project the patterned beam onto a target portion of thesubstrate; a liquid supply system configured provide a liquid in a spacebetween the final element of said projection system and said substrate;and a measurement system configured to measure, not through said liquid,the location of a each of a plurality of points on said substrate.
 2. Anapparatus according to claim 1, wherein said substrate table comprises areference and said measurement system is configured to measure, notthrough said liquid, the location of said reference.
 3. An apparatusaccording to claim 2, wherein said measurement system is configured tomeasure and determine the location of each of a plurality of points onsaid substrate relative to said reference.
 4. An apparatus according toclaim 2, wherein said measurement system is configured to measure thelocation of said reference at an exposure position, where if saidreference is not measured through said liquid a correction is applied.5. An apparatus according to claim 1, wherein the measurement systemcomprises an alignment system configured to measure the location of eachof a plurality of alignment marks on said substrate.
 6. An apparatusaccording to claim 5, wherein said substrate table comprises a referenceand said measurement system is configured to measure, not through saidliquid, the location of said reference.
 7. An apparatus according toclaim 6, wherein said measurement system is configured to measure anddetermine the location of each of a plurality of said alignment marks onsaid substrate relative to said reference.
 8. An apparatus according toclaim 6, wherein said alignment system is configured to measure thelocation of said reference at an exposure position, where if saidreference is measured not through said liquid a correction is applied.9. An apparatus according to claim 1, wherein said measurement systemcomprises a level sensor configured to measure the height and/or tilt ofeach of a plurality of points on said substrate.
 10. An apparatusaccording to claim 9, wherein said level sensor is configured to measuresaid height and/or tilt by projecting a leveling beam onto the substrateand detecting said leveling beam as reflected by the substrate.
 11. Anapparatus according to claim 1, comprising an exposure station at whichsaid substrate may be exposed and a separate measurement station, saidmeasurement system being provided at said measurement station and saidsubstrate table being movable between said exposure and measurementstations.
 12. An apparatus according to claim 11, comprising a pluralityof substrate tables, each movable between an exposure station and ameasurement station.
 13. An apparatus according claim 2, wherein saidreference comprises a transmission image sensor.
 14. An apparatusaccording to claim 1, wherein said measurement system is configured togenerate a map of the substrate from the location of each of saidplurality of points and comprising a controller configured to controlthe position of said substrate, using the map, during an exposure ofsaid substrate through said liquid.
 15. An apparatus according to claim1, wherein said measurement system is configured to measure the locationof each of said plurality of points on said substrate at an exposureposition before said liquid is provided to said space, and comprising acontroller configured to control the position of said substrate usingthe location of some or all of said plurality of points during anexposure of said substrate after said liquid is provided to said space.16. An apparatus according to claim 15, wherein said measurement systemcomprises an off-axis alignment system to measure the location of eachof said plurality of points.
 17. An apparatus according to claim 15,wherein said measurement system comprises a level sensor configured tomeasure the location of each of said plurality of points by projecting aleveling beam onto the substrate and detecting said leveling beam asreflected by the substrate.
 18. A device manufacturing methodcomprising: providing a liquid in a space between a final element of aprojection system and a substrate; measuring the location of each of aplurality of points on a substrate using a measurement beam projectedfrom a measurement system but not projected through said liquid; andprojecting a patterned beam of radiation onto a target portion of thesubstrate using the projection system.
 19. A method according to claim18, comprising measuring, not through said liquid, the location of areference on said substrate table.
 20. A method according to claim 19,comprising measuring and determining the location of each of a pluralityof said points on said substrate relative to said reference.
 21. Amethod according to claim 19, comprising measuring the location of saidreference at an exposure position, where if said reference is notmeasured through said liquid, applying a correction.
 22. A methodaccording to claim 18, wherein measuring the location of each of aplurality of points comprises measuring the location of each of aplurality of alignment marks on said substrate.
 23. A method accordingto claim 22, comprising measuring, not through said liquid, the locationof a reference on said substrate table.
 24. A method according to claim23, comprising measuring and determining the location of each of saidplurality of alignment marks on said substrate relative to saidreference.
 25. A method according to claim 23, comprising measuring thelocation of said reference at an exposure position, where if saidreference is measured not through said liquid, applying a correction.26. A method according to claim 18, wherein measuring the location ofeach of a plurality of points comprises measuring the height and/or tiltof each of a plurality of points on said substrate.
 27. A methodaccording to claim 26, comprising measuring said height and/or tilt byprojecting a leveling beam onto the substrate and detecting saidleveling beam as reflected by the substrate.
 28. A method according toclaim 18, comprising an exposure station at which said substrate may beexposed and a separate measurement station, said measuring of thelocation of each of said plurality of points being performed at saidmeasurement station and moving said substrate table between saidexposure and measurement stations.
 29. A method according to claim 28,comprising a plurality of substrate tables and moving each of saidtables between an exposure station and a measurement station.
 30. Amethod according to claim 18, comprising generating a map of thesubstrate from the location of each of said plurality of points andcontrolling the position of said substrate, using the map, during anexposure of said substrate through said liquid.
 31. A method accordingto claim 18, comprising measuring the location of each of said pluralityof points on said substrate at an exposure position before said liquidis provided to said space, and controlling the position of saidsubstrate using the location of some or all of said plurality of pointsduring an exposure of said substrate after said liquid is provided tosaid space.
 32. A method according to claim 31, comprising measuring thelocation of each of said plurality of points using an off-axis alignmentbeam.
 33. A method according to claim 31, comprising measuring thelocation of each of said plurality of points by projecting a levelingbeam onto the substrate and detecting said leveling beam as reflected bythe substrate.